Printed circuit boards (PCB) have long been used as the base for sophisticated electronic systems. An electrically insulating sheet, originally phenolic impregnated fabrics and now generally fiberglass reinforced resins, is coated with copper cladding and has appropriate patterns etched into the cladding. In years past, most electronic components had wire leads that extended through holes drilled into the cladding pattern and filled with solder to make the required connections. More recently, surface bonding of relatively short leads to the cladding has become common, allowing for high-speed robotic placement of components.
Today, electronic devices are increasingly miniaturized and it has become desirable to mount component on both sides of a PCB. However, there are a number of problems associated with installing parts on the second side after components have been mounted on the first side. The board cannot be held flat with downwardly projecting components of various sizes and thicknesses mounted on the lower side. This problem is most acute when solder paste is to be printed on the second side. Holding the PCB flat and level under a paste application stencil during solder paste application and then during component placement is very difficult.
In high production run circumstances, aluminum plates or similar materials have been machined out in a pattern corresponding to the topography of the first side of the PCB with components installed. This approach is not practical for manufactures or subcontractors producing a limited quantity of a very great number of PCB configurations, each requiring its own “hogged-out” support plate.
Supports have also been made by casting a plaster-like material into a mold corresponding to a particular PCB to form a support having pockets for receiving the components on the downwardly extending board side.
While effective where a large number of identical boards are to be manufactured, these methods are not cost effective where only a few boards are to be made or where custom boards are being manufactured.
A number of different devices having a plurality of adjustable length upstanding fingers have been developed to support an irregularly shaped article. Typical of these devices is the device for supporting parts during machining as described by Barozzi in U.S. Pat. No. 4,936,560, the casting support device describe by Godding in U.S. Pat. No. 4,200,272, and the core support system described by Bourassa et al. in the U.S. Pat. No. 3,530,994.
While these supports are generally effective for their intended purposes, they are overly complex, and do not always provide positive support across the supported object.
A circuit board support system using a plurality of spaced-apart, parallel, upwardly extending pistons is described by Fadiga et al. in U.S. Pat. No. 3,942,778. This support is used to press the back of the boards against test sensors. Since the pistons are not lockable to match a particular PCB, each succeeding board must be pressed down against the pistons, risking damage. Further, the system is not readily useful in installing components on the back of the board, since the pistons continue to press upwardly so that the board may not lie truly flat and may move during back-side component installation.
Thus, there is a continuing need for improved supports for holding a substrate, such as a printed circuit board, having components mounted on one side while additional components are installed on the opposite side, that will support the board in a precisely level position, that will provide strong, consistent support for the board during second surface stencil printing and soldering operations, that can easily be locked in the support position appropriate to a series of similarly configured boards, that precisely indexes board edges and that is easily unlocked and reconfigured for other boards.
U.S. Pat. No. 5,897,108 describes a series of plates with aligned holes through which spring loaded pins protrude. A top and bottom plate hold the pins in position laterally, while a middle plate is moved out of alignment to clamp the pins, holding the pins in vertical alignment to the underside topology of the substrate.
U.S. Pat. No. 5,897,108 further describes a means for moving the middle plate out of alignment, and then describes two ways to do so.
The first way is to move the middle plate by applying force against the edge of the middle plate with screws. Typically, one of the middle plate's edges protrudes out further than the edges of the top and bottom plates while the pin holes are in alignment. Screws can then be placed through the frame that surrounds the three plates. The screws push against the middle plate's edge and move it out of alignment. While this method is simple, it will eventually permanently deform the middle plate at the point where force is applied.
The preferred way described in U.S. Pat. No. 5,897,108 is to use a cam, which uses holes in the top and bottom plates to anchor the cam at its ends. The cam's lobe is then pushed against the middle plate, thereby moving it out of alignment when the cam is rotated. This method is not as robust as it could be, in that the anchor points of the cam wear easily because the plates are very thin (typically 0.048″ thick) and made from aluminum.
FIGS. 1-3B illustrate a version of U.S. Pat. No. 5,897,108 that is currently used. It should be noted that while the patent shows a system with a wide array of pins, the units can be made in strips 2, typically three pins wide, having front-end rail 4, rear-end rail 6, and side rails 8. The strips 2 are then stacked next to each other to make up wider arrays as manufacturing operations, machine conditions and substrate size dictate (compare the first figure of the patent with FIG. 1 of the current method).
While this version still uses a cam 28, it no longer uses the top and bottom plates 22 and 18 as anchor points. Instead, a cam slot retainer (the cam receptacle) 32 is anchored to the middle (clamping) plate 20 with four screws 38. One end of the cam is then fixed with a cam block housing (the cam hub) 24, which in turn is anchored to the very bottom (base) plate 10 of the system. The cam block housing's position can be adjusted back and forth with an adjustment screw 26, and four screws 40 then hold the block in position to the bottom, as seen in FIGS. 2 and 3. Elastically compressible elements, such as pin springs 14, are positioned in recesses formed in base plate 10 and preferably separated by plastic insert plates 12. These compressible elements exert an upwardly directed biasing force on pins 16, which protrude through the apertures 36 in the bottom, middle and top plates. With the cam's hub 24 anchored at the base 10, the cam lobe rotates inside the cam slot retainer 32, which in turn moves the middle plate 20 out of alignment, thereby distorting and locking pins 16 in position. A T-handle Allen wrench 34 is used to engage the cam 28 and rotate it to its locked and unlocked positions.
There are also two plate return springs 30 that push against the front-end rail 4 and the cam slot retainer 32. When the pins 16 are unlocked by rotating the cam 28 to the unlocked position, the middle plate 20 is moved to its aligned position relative to the top and bottom plates 22 and 18 with the help of the springs 30. The cam slot retainer 32 was designed, not as a true cam receptacle (although it could have been, thus alleviating the need for the springs), but to have some “slop” built in to allow for manufacturing tolerances in the system. The springs 30 are therefore necessary to help the middle plate 20 to return to an aligned position. In addition, the pins 16, being made of an elastic material, help move the middle plate 20 to its aligned position when the plate is unlocked, but the springs 30 are used to finish the movement.
Not shown in the figures is a ball plunger that goes through the front-end rail 4 and screws into the cam slot retainer 32, pressing against the cam 28. The cam 28 has a dimple in its side so that when the cam 28 is in the locked position, the plunger engages the detent and keeps the cam 28 in the locked position.
The cam 28, the cam slot retainer 32 and the cam block housing 24 are expensive to manufacture (approximately $44.00 per unit). Furthermore, additional tooling is required (the Allen wrench—a $3.00 item). The present invention provides simpler methods, equally robust, which improve on the patented and current methods of manufacture.